With continuous development of wireless communications system technologies, to implement communication, a D2D communication mode may be further used in addition to a conventional cellular communication mode. In the cellular mode, user equipment (UE) communicates with another UE using a base station. However, in the D2D communication mode, UE directly communicates with another UE such that not only a radio spectrum resource is saved, but also pressure on a core network is reduced. Therefore, a D2D technology becomes an important technology that is currently being standardized in the 3rd Generation Partnership Project (3GPP) standard and that can be compatible with the 5th Generation (5G).
Currently, enhanced D2D (eD2D) is a topic being studied in the 3GPP Long Term Evolution (LTE) system release.13 (Re1.13). In this topic, UE-to-network relays are an important research orientation. The UE-to-network relays are classified into two scenarios, communication and discovery. For the two scenarios, a specific communication process, resource allocation, data transmission, and the like need to be designed and studied correspondingly.
Currently, how to find UE in network coverage that can be used as relay UE to transmit information from UE out of network coverage to a base station becomes an urgent problem to be resolved.